To shorten a construction period by forming an intermediate wall of a precast material when forming a branch part in an existing tunnel, and reinforcing a remaining side wall of an expanded width part by grout injection via the precast material.

In work for connecting a branch part to a tunnel 3 by forming the branch part in an existing tunnel 1, first of all, a protector is arranged in the tunnel 1 to secure traffic, and an expanded width side wall is broken up to be excavated to execute a lining. The protector is removed, the traffic is switched to the tunnel 3 side, an intermediate wall joining member 7 and an intermediate wall fixing part 8 are arranged, a steel precast material 10 is fixed, a precast material 11 is fixed to a surface, a concrete part 6a is arranged on the reverse to be formed as an intermediate wall, and a filler 21 such as air mortar is filled to close the tunnel 1. A precast material is arranged on a surface to a remaining side wall of an expanded width part having no intermediate wall, and grout is injected into clearance to be reinforced. Thus, a construction period can be shortened without using a form.